Adding Widget Button Toolbar component

- Importing source code in `index.html`
- Updated `app.js` to add widget button toolbar as dependency for style guide app
- Updated `controllers.js` to add widget button toolbar to `$scope.uiComponents`
- Added demo html file
- Added `widget-button-toolbar.js` to libs
- Including widget button toolbar in `bower.json` under the other components listed for `widget-settings-ui-components` repo